Aalto – Tartu Summer Course: Antenna Basics

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Aalto – Tartu summer course: Antenna basics Jari Holopainen, Joni Lappalainen Department of Electronics and Nanoengineering Aalto University, School of Electrical Engineering June 12, 2017 What is an antenna? • Definition: An antenna is (part of ) a device that is designed to transmit and receive radio waves; in transmission an antenna converts a guided wave into a free space wave, and in reception vice versa. • Why the following devices are not antennas: a) laser pointer, b) human eye, c) electrical power lines, d) TV remote control? • In your opinion, which words in the definition should be “highlighted” or “bolded” and why? Antenna radiation Alternating current (AC) in the antenna structure induces an oscillating electric and magnetig fields – i.e., radio waves – that propagate to the surrounding space with the speed of light. Vertical dipole antenna and its electric field strength (V/m) Antenna radiation Alternating electric current in the antenna structure induces an oscillating electric and magnetig fields – i.e., radio waves – that propagate to the surrounding space with the speed of light. Horn antenna and electric field strength (V/m) Far-field radiation pattern Describe, how each antenna radiates to the surrounding space. vertical dipole horn Spherical coordinate system Are you familiar with the spherical coordinate system? Why antennas are often studied in the spherical coordinate system? Far-field radiation pattern Antennas radiate more to certain direction than the others – this feature is described with the radiation pattern Radiated power density (W/m2) presented in the 3D spherical coordinate system vertical dipole horn Main lobe Far-field radiation pattern Antennas radiate more to certain direction than the others – this feature is described with the radiation pattern The very same presented in the 2D polar plot Antenna directivity Directivity describes in one number how much the antenna radiates to the main lobe compared to all directions on average maximum power density (W/m 2 ) D average power density (W/m 2 ) S S  max  10 log max dBi S ave S ave The larger the number, the more the antenna radiates into a certain direction Antenna directivity, example A horn antenna has the main lobe with both cuts halfpower beamwidths (HPBW) 30o. Approximate the directivity. (A: about 17 dBi) S max maximum power density (W/m 2 ) S max D   10 log dBi 2 average power density (W/m ) S ave S ave Antenna gain and efficiency The gain (G) is the directivity (D) and the efficiency (η) multiplied: G   D • • • Often given in desibels: GdBi = 10 log G dBi Efficiency is 0 < η < 1 and it includes all the possible non-wanted losses (to be discussed in the following slides) Despite the name ”gain”, the radiated power of the antenna does not increase! (because an antenna is a passive component!) Voltage reflection from a mismatched impedance Voltage (V) What happens and why in the interface of two impedances Z0 = 50 Ω and ZL = 16.7 Ω ? Voltage reflection coefficient, ρ The voltage reflection due to the impedance mismatching is presented using the voltage reflection coefficient, ρ voltage reflection coefficient: Z A  f   Z0  Z A  f   Z0 Voltage reflection coefficient, ρ The voltage reflection due to the impedance mismatching is presented using the voltage reflection coefficient, ρ example  Z A  f   Z0 Z A  f   Z0 Typically presented in dB: 2  (dB)  10 log   20 log  At the resonant frequency, an antenna ”takes in” the largest amount of power (The resonance itself is not needed for the radiation but it helps for the efficient power transfer between the feed cable and the antenna.) Reflection coefficient, ρ  Z A  f   Z0 Z A  f   Z0 How much (%) power we lose when the reflection coefficient is… a) -3 dB ? b) -6 dB ? c) -10 dB? d) -20 dB? Efficiency; definition of different powers Pavailable = power available from the generator Pin = power going in the antenna from the feed Preflected = power reflected from the input of the antenna Pradiated = power radiated into the far field Ploss = loss power due to the resistive losses of the antenna structure Matching efficiency Pin power accepted by theantenna 2 m    1  power available in the input Pavailable 0  m  1 Radiation & total efficiency  rad  Pradiation power radiated into the far field Pradiation   power accepted by the antenna Pin Pradiation  Ploss tot Pradiation    rad  m Pavailable tot  rad (often called also as ”antenna efficiency”) 0   rad  1 Effect of the antenna on radar operation? Consider and list, how the antenna affects the operation/performance of a radar. Extra: reflection from the ground How the radiation pattern changes due to a ground reflection? Conductive ground